Conventionally, in an exposure apparatus used in the manufacture of semiconductor devices, the XY stage is mounted atop a vibration-free device. Generally, these vibration-free devices are of two types: passive vibration-absorbing systems, using air springs, coil springs, vibration-proof rubber, or the like, to attenuate vibration, and active vibration-damping systems that drive an actuator such as a voice coil motor.
However, vibration-free devices equipped with the passive vibration-absorbing systems described above, though they may be able to attenuate vibrations transmitted through the floor of the apparatus to some extent, are unable to effectively attenuate vibrations generated by the movement of the XY stage itself.
Moreover, the passive vibration-absorbing systems described above must strike a good balance between insulation from vibrations propagated through the floor of the apparatus, which is to say vibration-free performance, and the ability to suppress vibration due to movement of the XY stage itself, that is, damping performance.
In recent years, an active damping system has been suggested, in which, in order to prevent the reaction torque to the driving of the XY stage from forcibly vibrating the isolation table, a counter-mass is driven in a direction opposite to the direction in which the stage is driven, so as to offset the drive reaction torque by the movement of the load so that vibrations do not reach the isolation table, the object being to provide a trade-off between vibration-free and vibration damping performance.
An example of the structure of a conventional active damping device is shown in FIG. 1. The active damping device shown here is designed so that three separate control units, consisting of controllers 31-41 for air spring supports 1, 8, controllers for the stage 18-22, and counter-mass drive controllers 25-29, operate independently. Hereinafter, for convenience, a direction in which a stage 16 is driven defines the X axis, a direction vertical thereto is the Z axis, a direction through the center of gravity of an isolation table 15 orthogonal to a direction of movement of the stage 16 is the Y axis and an axis of rotation around the Y axis the (ωY) axis.
The isolation table 15, which mounts the stage 16 and a counter-mass 23, is supported by air spring supports 1, 8. The air spring supports 1, 8 have air springs 3, 10, pressurized mechanical springs 5, 12, and viscosity elements 6, 13 for attenuating viscosity in order to eliminate vibration from the floor. Servo-valves 2, 9 control the supply and exhaust of air, which is the fluid body that operates the air springs 3, 10. Movement of the air springs is monitored by position sensors 4, 11 so as to measure changes in the position of the isolation table 15. The detection readings from the position sensors 4 and 11 are input to displacement amplifiers 30 and 31, respectively, and from the displacement amplifiers 30 and 31 are input to a displacement analyzer 32. The displacement analyzer 32 then analyzes the readings into a vertical (Z axis) displacement component and a rotational displacement (ωY) component about the Y axis using calculations on a 2×2 matrix having as its elements the displacement and rotation (angle of rotation) of the air springs 3, 10, with the vertical and rotational displacement components forming negative feedback for calculating a deviation signal to be input to PI compensators 33, 34.
Next, acceleration readings of the isolation table 15 measured by acceleration sensors 7, 14 are input to an acceleration analyzer 37 after having been fed respectively through each of filters 35 and 36, which have predetermined appropriate gain and time constants. The acceleration analyzer 37 then analyzes the acceleration readings into a vertical (Z axis) acceleration component and a rotational (ωY axis) component about the Y axis, using the two inputs from the acceleration sensors 7, 14, with the vertical and rotational displacement components forming negative feedback for calculating a deviation signal to be input to a thrust allocator 38.
A target position setting unit 41 then sets a target position for the displacement of the isolation table 15 in the vertical and rotational directions, with a deviation signal between the set target value and the displacement analyzer 32 output signal being input to the PI compensators 33, 34. The deviation signal between the PI compensators 33, 34 output signals and the acceleration analyzer 37 output signal are then input to the thrust allocator 38.
The thrust allocator 38 then allocates command values for controlling the air springs in the vertical (Z axis) direction and the rotational (ωY) direction about the Y axis as drive command values for the air springs 3 and 10. The drive command values so allocated are each converted into drive currents by the amplifiers 39 and 40 and by the servo valves 2 and 9 adjusting the pressure inside the air springs 3 and 10 so that the isolation table 15 can be held at a desired position set by the target position setting unit 41 without steady-state deviation.
The PI compensators 33 and 34 each acts as displacement control compensators in the vertical direction (that is, along the Z axis) and in the rotational direction about the Y axis (that is, along the ωY axis), respectively. (The “P” element of the PI compensator proportionally calculating the extent of adjustment, the “I” element compensating so as to perform integral calculations with respect to the extent of adjustment.)
The foregoing comprises control of the air spring supports 1, 8 for the isolation table 15.
Position readings from a position sensor 17 that measures horizontal displacement of the stage 16 mounted atop the isolation table 15 are input to a displacement amplifier 18. A deviation signal between the output signals from the displacement amplifier 18 and the output signals from an integrator 22 that integrates velocity readings output from a target velocity generator 21 and generates stage 16 target position data is then input to a PID compensator 19, so that the extent of adjustment amplified by the amplifier 20 is applied to the stage 16.
Here, the “D” element in the PID compensator 19 compensates so as to perform differential calculations. The stage 16 target position command is obtained by integrating the target speed generated by the target velocity generator 21 with the integrator 22, thus completing control of the stage 16.
Further, position readings from a position sensor 24 that measures horizontal displacement of the counter-mass 23 mounted on the isolation table 15 are input to a displacement amplifier 25. A deviation signal between the output signals of a displacement amplifier 25 and the output signals from an integrator 29 that integrates velocity readings output by a target velocity generator 28 and generates counter-mass 23 target position data is then input to a PID compensator 26, so that an extent of adjustment amplified by an amplifier 27 is applied to the counter-mass 23, thus completing drive control of the counter-mass 23.
FIGS. 2A, 2B, 2C, 2D, 2E and 2F are diagrams illustrating stage and counter-mass target values, velocity patterns and acceleration patterns.
FIGS. 2B and 2E show the time (the X axis) of the target velocity generated by the target velocity generators 21 and 28 with respect to the driving of the stage 16 and the counter-mass 23. The velocity pattern shown here is known as a trapezoidal velocity pattern, having (from left to right in the diagram) an acceleration region, a constant velocity region and a deceleration region.
FIGS. 2A and 2D show the target acceleration, which is a differential of the target velocity with respect to the driving of the stage 16 and the control counter-mass 23. As can be seen, in regions corresponding to the acceleration and deceleration regions shown in FIGS. 2B and 2E, the direction of acceleration is inverted. In the constant velocity region, the acceleration is zero.
FIGS. 2C and 2F show the target position, which is an integral of the target velocity with respect to the driving of the stage 16 and the counter-mass 23. The position-controlled stage 16 and counter-mass 23 gradually approach the target positions (indicated by the dotted lines) and are positioned. The extent of the movement to these target positions matches the surface areas of the velocity patterns shown in FIGS. 2B and 2E.
The drive reaction torque arising during acceleration and deceleration of the stage 16 can be offset by driving the counter-mass 23 in the opposite direction from the stage 16, so that the drive reaction torque imparted to the isolation table 15 is negated by the moment change due to the load movement. As a result, the drive reaction torque caused by the driving of the stage 16 can be prevented from reaching the isolation table 15.
However, when the stage 16 and counter-mass 23 are initialized, that is, when the initial settings are installed or the control parameters updated, etc., the stage 16 and the counter-mass 23 must be driven independently. But when the stage 16 and the counter-mass 23 are driven independently, the drive reaction torque and load movement moment so generated are not offset but are transmitted directly to the isolation table 15 as external forces. Moreover, if the stage 16 and the counter-mass 23 are moved at high speed, the isolation table 15 is agitated by the drive reaction torque and the load movement-generated moment and may exceed permissible drive range limits.
In order to prevent such over-extended movement of the isolation table 15, the stage 16 and the counter-mass 23 can be driven at a reduced speed so as to achieve the desired initializations. However, when employing an active damping apparatus in a semiconductor exposure apparatus to isolate the stage, the impact of the above-described initializations can result in reduced through-put of the apparatus.